A U.S. federal judge's ruling has thrown the government's system of military tribunals for suspected terrorists into legal doubt. The judge ruled that the Bush Administration acted improperly in summarily denying prisoner of war status to a man captured in Afghanistan.

The tribunal trying Salim Ahmed Hamdan for alleged war crimes came to an abrupt halt after a federal judge ruled that Mr. Hamdan had been improperly denied any opportunity to demonstrate he is a prisoner of war.

In a 47-page opinion issued in Washington late Monday, U.S. District Judge James Robertson said Mr. Hamdan's military trial at the U.S. detention center at Guantanamo Bay, Cuba, was illegal. Judge Robertson ruled that Mr. Hamdan, an alleged al-Qaida member and Osama bin Laden's driver, cannot be tried until he has a chance to appear before a board to appeal for POW status.

The ruling only directly affects Mr. Hamdan. But legal scholars say it has the practical effect of throwing the tribunal system into doubt.

The Bush Administration has designated the men held at the detention facility at Guantanamo Bay, Cuba, as "enemy combatants," denying them special protections they would receive as prisoners of war. It set up special military tribunals to try some of the 550 detainees captured in Afghanistan for war crimes. But Judge Robertson said the administration violated the Geneva Conventions, which are international treaties governing the treatment of POWs.

A U.S. Justice Department spokesman denounced the ruling, saying it gives terrorism the same legal status as waging legitimate war. He said the government would appeal it to a higher court.

Conservative legal scholars also criticized the ruling. Richard Samp, chief counsel of the Washington Legal Foundation, said the ruling represents unwarranted intrusion by a liberal judge into military matters.

"These sorts of judicial interventions are what give the federal judiciary a bad name," said Mr. Samp. "The courts ought to be deferential to the military when it comes to military affairs, and we ought to be more concerned about fighting the war on terrorism than we are in creating new constitutional rights for people who are not U.S. citizens and have never been in this country."

But Eugene Fidell, president of the National Institute of Military Justice, says there are cases where the judiciary does intervene on military matters. He says the ruling reflects a frustration by some judges over the administration's handling of the detainee issue.

"All these cases I think are heading ultimately to yet another showdown, this term or next term, in the Supreme Court of the United States," said Mr. Fidell. "Now one swallow, as the saying goes, does not a summer make. But I think what you are seeing is a pattern of frustration on the part of the federal district judges with the administration's policies, with the administration's construction of the law, and with the pace of events."

The tribunals, also known as military commissions, were last used during World War Two to try German saboteurs. Human rights groups have criticized the system as unfair and even some U.S. allies, such as Britain and Australia, which have some of their own nationals detained at Guantanamo.